A conventional engine starter for use in starting up a vehicular engine has been constructed as shown in FIG. 1.
The conventional starter 1 shown in FIG. 1 comprises a d.c. motor 2, an over-running clutch mechanism 4 slidably mounted over an armature rotary shaft 3, a planetary gear speed reduction unit 5 for reducing and transmitting the rotating speed of the armature rotary shaft 2a of the d.c. motor 2 to a clutch outer member 4a of the over-running clutch mechanism 4, and a shift lever 8 engaged at its one end with a plunger rod of a solenoid switch assembly 6 which is disposed on one side of the d.c. motor 2 for slidably moving the over-running clutch assembly 4 and at its the other end with an annular member 7 mounted to the overrunning clutch mechanism 4.
However, the conventional starter 1 needs the shift lever 8 for sliding the over-running clutch mechanism 4 on the output rotary shaft 3, and since the bi-axial arrangement, in which the electromagnetic switch assembly 6 for actuating the shift lever 8 and supplying electric power to the d.c. motor 2 is disposed on the side of the d.c. motor 2, is employed, strict limits are imposed on engine layout during the design of the vehicle.
In order to eliminate the above problem, it has been proposed to position the solenoid switch on one of the axial end portions of the d.c. motor to form the starter into a simple configuration such as an elongated cylinder. According to this proposition, the basic construction is such that the armature rotary shaft is made hollow and the plunger rod of the solenoid switch assembly which has been used to operate the shift lever is elongated to extend through the inner passage of the armature elecromagnetic core to reach the output rotary shaft. The starter having such construction is referred to as a coaxial starter since the armature rotary shaft of the d.c. motor and the rod of the solenoid switch are coaxially aligned.
However, while the above-discussed coaxial starter is a simple elongated cylindrical member, the overall axial length is significant if the solenoid switch assembly is simply attached to the rear end of the d.c. motor. On the other hand, if the solenoid switch assembly is simply reduced in size, it is difficult to maintain the necessasry magnetic attractive force. Therefore, the development of a coaxial starter of the above structure with a shorter overall length has been expected.